Multioffice telephone system



July 16, 1929. j E AX' 1,721,226

MULTIOFFICE TELEPHONE SYSTEM Original Filed June 3. 1922 8 sh ts-sheet 1 n'usn m: L'Zareuce E-L1Jm5x I.

July 16, 1929. c. E. LOMAX MULTIOFFICE TELEPHONE SYSTEM Original Filed June is. 1922 a Shets-Sheet 2 I I I I I I J nuan m: Ilarence E-Lmmax mumn m I m I I I I I I I July 16, 1929. c. E. LOMAX MULTIOFFICE TELEPHONE SYSTEM Original Filed June 5. 1922 8 Sheets-Sheet 3 Im rEI-LZ/Jr I? 15251-11315 Elmmax My 16 1929. c. E. LOM AX 1,721,226

I MULTIOFFICE TELEPHONE SYSTEM Original Filed June 3. 1922 a Sheets-Sheet 4 -Inuen DI'-- Ilarauce ELUmaX July 16, 1929. c, LOMAX 1.721226 MULTIOFFICE TELEPHONE SYSTEM Origifial Filed June 5. 1922 s Sheets-Sheet -Iuuen 01 Q Elazsnue E-Lumax Jui 16, 1929. E, AX 1,721,226

MULTIOFFICE TELEPHONE SYSTEM Original Filed June 3, 1922 a Sheets-Sheet 6= Flaranceflllumax v 5 July 16, 1929. c LOMAX 1,721,226

I E MULTIOFFICE TELEPHONE SYSTEM Original Filed Junefi. 1922 8 Sheets-Sheet 8 ImJEu LIII' Ilazzance E LUTHEX Patented July 16, 1929.

UNITED STATES PATENT OFFICE.

CLARENCE E. LOMAX,-OF CHICAGO, ILLINOIS,IASSIGNOR, BY IMESNE ASSIGNMENTS, T0

AUTOMATIC ELECTRIC INC., OF CHICAGO, ILLINOIS, A CORPORATION :OF DELA- WARE.

MULTIOFFICE TELEIPHONE SYSTEM.

Application filed June 3, 1922, Serial No. 565,727. Renewed March 7, 1927.

namesor numbers of exchanges in a multioilice system, which according to the Strowger plan would be assigned to oflices in one geographical district, may be assigned to offlees in more than one geographical district.

A particular object of the invention is to provide novel and improved switching equipment for accomplishing the above results andfor increasing the flexibility of the inter-oflice trunking in large multi-oflice systems.

It is especially desirable to accomplish the above results in view of the fact that a large number of multi-oflice manual systems are now being changed to automatic operation and'when a transition of this kind is made it isadvisable to retain the old manual exchange names as well as thesubscribers numbers, wherever possible, in order to avoid undue confusion. In accordance with this plan, when the automatic system is installed the calling devices-have letter combinations in the finger holes in addition to the usualnumbers so that the ofiice names, thatis the first two or three letters of each oflice name may be dialled by calling subscribers, these letters forming part of the directory numbers of the subscribers. lVhen a large multi-office manual system is changed to automatic operation it is usually necessary to use the first three letters of the- .manual exchange names aspart of thedirectorynumbers of the subscribers in order toisecure properse-' v lection of exchanges, the remaining part of the number consisting of four digits. Under these conditions it w ll be appreciated that,

i as in the majority of cases the manual'exchange names are promiscuously assigned, situations are encountered in which there is a confliction between the first one or two letters in the names of exchanges in different disti'icts,-the letters of these names appears ing in the same finger holeson the dial, and

it becomes necessary to provide trunking arrangements of the kind indicated above to.

handle connections to these exchanges. The present invention is designed to take care of trunking connectionsof this character very economically.

There are a number at other abjects the invention, which together with those eiiumerated' above, will be described in detail in the technical description which is to follow, taken in conjunction with theacco'm'panying drawings forming part of this specification.

Referring now to the drawings comprising Figs. 1-10, inclusive, Fig. 1 is a'mapot certain of exchanges in the system and the trunk lines interconnecting them. Fig. '2 is aview ofthe calling device dial. Fig. 3 shows the manner inwhich the detailed circuit 'drawings are to be placed inorder to follow the descriptionfof theoperati'on. "Fig. 4' shows the trunking and switches involvedin typical connections shown in accordance with i the map of exchanges, Fig. 2. Fig.510, inclusive, show by means of the conventional'circuit diagrams enough of the apparatus to enable the invention to be readily explained and understood. 7

The invention is, of course, applicable to various sizes of multi-oifice telephone systerns; however, in the presentc'ase it has been illustrated in 'connection with a seven digit system, or one the ultimate capacity of which does not exceed. ten million line's. In this trict will be connected to every other main exchange in the district by direct trunks; thus each district forms a complete trunkingfnetwork." To handle calls between the various districts each main olfi'ce'in every district is connected by trunk lines to. one of the nearest olfices in every other district of the system, theselast mentioned main exchanges serving as switching centersfor inter-district calls. Arriving atthese so called switch ng centers, the incoming'calls 'are then routed through the inter'ofiice trunkings'ys tem in the same manner as a local district call. Since the first three letters dialled determine'to what exchange the call is going, if the first two'let{ ters of offices in-difterent districts are the same, i s will be necessary to use trunk lines extending tothe districtsin wh ch these offices Located when a, call is made ta either at fice, and to release one connection when the third digit is dialled, which digit finally dotermines to what exchange the call is goin In describing the drawings the general lay out of the system will first be explained with reference to the map of exchanges, Fig. 1. In this figure, there are shown three districts, which have been indicated on the drawings by the reference characters A, B, and C. All the main exchanges in districts A, B and C, are connected together by trunk lines, while each main exchange has trunk lines to one exchange in every other district; thus, the main ofiice Lombard in geographical district B has trunk lines extending to all the main exchanges in district B, and to the main ex changes Grove and Main, in geographical districts A and C. Only a few of the main exchanges in each geographical district have been shown, but it will be understood that there may be as many as ten main offices in Y each district. The arrow heads on the trunk "lines indicate whether they are a group of one way trunk lines or a group of two way trunk lines. Of course, for these two way trunk l1nes may be substituted two groups of one way trunk lines. Only a portion of the system has been illustrated, and it will be understood that there will be a large number of other districts in the system. By referring to the names of the exchanges in the various districts, Fig. 1 and the calling device dial,

Fig. 2, it will be seen that there is a confliction in the names of the exchanges Larabee and '2 Main in district C and Maburn in district A.

This confliction will serve to bring out all the features of the invention.

Referring now to' the trunking diagram shown in Fig. 4, the various trains of switches involved in typical connections in the trunking situation shown in the map, Fig. 1, are illustrated. To give a general understanding of the operation of the system, the

manner in which callsare extended from the main office Center to the varlous other offices, Wlll now be descrlbed wlth reference to Fig. 4.

For the purpose of the description, it will first be assumed that the subscriber at substation A in the Center ofiice, desires to call a subscriber at substation A in the Larabee oiiice. In order to accomplish this result, the

calling subscriber will remove his receiver and operate his calling device for the number LARabee-i67x, the numerical equivalent of Larabee being 517, as may be seen from the drawings. The letter x, as used in ,idle trunk line leading to a selector repeater,

such as 1E. The selector repeaters such as 1E have associated with them switching discriminators such as the switching discriminator SD shown in the drawings and also the secondary line switches C and C The selector repeater 1E has an extra set of bank contacts and cooperating wipers. When the first digit L is dialled both sets of wipers of the'districts A and C, respectively, for con-- nections from Center exchange. It will be assumed that the trunk lines leading to the second selector 2E in the Grove exchange and the second selector QB in the Main exchange are the trunk lines selected. In response to the second character A called on the calling device at the calling substation, the

second selectors 2E and 2E in the Grove and Main exchanges, respectively, are operated'to the first level of bank contacts. These selectors then operate to select trunk lines leading to idle third selectors. It will be assumed that the second selector 2E selects the third selector 3E in Grove exchange, and that the second selector 2E in the Main ofiice selects the third selector 3E in the same office. The switching discriminator SD is also operated in accordance with the second digit of the ,called number and its wipers are raised opposite the first level of bank contacts. In response to the next character R,'the selector repeater 1E operates to repeat the digits to the third selectors 3E and 3E in the Grove and Main exchanges, respectively, and the wipers of these switches are raised opposite the seventh levels of bank contacts and are then rotated in search of idle trunk lines. In the Grove exchange, the seventh level of the third selectors are dead and consequently the trunk selecting operation of the third selector 3E is without function. The third selector 3E in the Main exchange operates to select a trunk line extending through a repeater such as R to an idle incoming fourth selector in theLarabee exchange such as the selector 41E. 7 The switching discriminator SD is also operated in accordance with the character R and as this character determines that the Larabee and not the Maburn ofiice is being called the switching discriminator SD causes the trunk extending by way of the secondary line switch C to be released. This operation also'releases the switches 3E and 2E in the Grove exchange. In response to the four remaining digits of the called number, 467x, the fourth selector 4E, a fifth selec= tor 5E andaconnector H in the Iiarabee exchange are operated to complete'the connection to the line of the substation A theline relay of'the selector repeater 1E repeating subscriber at Center exchange desires to call a subscriber at substation A inlthe Lomthe impulses to the'linerela'y of the repeater R which intu'rn repeats these impulses to the switches 4E, 5E, and H; i

It will now be assumed that the calling subscriber at substation A instead of de siring to connect with a'subscriber at substation A in the Larabee oflic e, desires to connect'with the line of the substation A in the main oflice', whose number is MAInl7-35x, the numerical equivalent of; MAI beinghll as is shown just below the office name inFig. at. In this case in 'response to the removal of the receiver and the dialling of the lettersMA the connection will be extended as before. In response to the thirdcharacter I the wipers of the third selectors 3E and 3E, instead of being raised to the seventh levels of bank contacts as was the case in the'preceding connection, are raised to the first levels and are then rotated in search of idle trunk lines. As before, the third selector 3E inthe main-ofiice is the only one that has a useful function, that being to select an idle fourth selector such as lEh' The switching discriminator SD in operating in response to the third character I, causes the release of the trunk line to Grove andthe operated switches therein in the samefmanner' as has already been described. ing digitsofthe number 735x, the fourth selector 4 ,121; fifth selector 5E and a connectorH are operatedto extend the connection" to theline of the substation A It will now be assumed that the'calling subscriber instead ,of desiring to call a subscriber at substation A in the Main office in district C, desires to call a subscriber at substation A in the Maburn ofiice of district A, whose number is MABurri-685txj Un-' der these conditions the character B will be called for the third digit and the' wipers of the third selectors 3E and 3E in the Grove'and Main ofl ices, respectively, will be raised opposite the second levels of bank contacts. In this case the operation of the selector 3E in the Main oflice is without func tion as the switching discriminator- 8Din.

line shown' in the drawings extending through'therepeater R and terminating in an incoming fourth selector in theMaburn office. In response to the four remaining-digits 634x of the called number,'the

incoming fourth selector 4E, a fifth selector In response to the four remain- In operating in response to the third 5E and a connectorH areoperated to com- I plete the connection to theline of the substation A ;It will now be assumed that the calling bard exchange, whose number is.LOMbard- 445x. In this case, after the first character L is dialled, which will be the same as the digit M,being in the same hole on the calling device, the calling subscriber will dial the character 0 whereupon the second selectors 2E and 2E in the Grove and Main exchange, respectively, respond and their out functionin the present instance as the c discrimiexchange Lombard is being called. In re sponse to the third digit M the wipers of the switching discriminator SDare rotated in on the fifth bank contact set in the tenth level.

The switching discriminator SD now causes, the trunk lines extending to the Grove and Main 'ofiices to be released as Well as the ing subscriber, to completethe desired connection. r

It will now be assumed that; the subscriber at substation A desired to extend a connection to a subscriber at substation 1A in the Grove exchange, the directory numberof this subscriber being GROVe G ISX. In response to the removal .of the receiver the primary line switch C operates to select an idle selector repeater, which, we will assu neiis the selector repeater 1E. When the calling device is operated inaccordance w th the character G" the wipers of the selector repeater 1E are raised opposite the fourth level of bank contacts and are then rotated insearchof an idle trunk line. It willbe assumed thatthe trunk line extending to the lineswitch CF is the one selected. The secondary lineswitch C now' operates to select a trunk lineextending to an idle incoming second selector in the Grove exchange. It will be assumed that theselector 2E 1n the Grove exchange 15 the oneselected. As there areno ofiices in the A district having the second letter R or one inthe same hole on the calling device dial, it is posthe fourth and .fifth levels Q1;ma nate; :6,,

I are brought opposite the seventh level of bank contacts and are then rotated in search of an idle trunk line. It will be assumed that the third selector 3E is the one selected and connected with by the second selector 2E It will be noted that it is possible to use one 'group of third selector switches such as 8E to handle connections to both the Grove and Maburn exchanges for the reason that there is no confliction in the third digit of the exchange names of district A, these being in separate finger holes on the dial. In response to the third digit O the wipers of the third selector 3E are raised to the tenth level and are then automatically rotated to select an idle trunk line leading to a fourth selector such as 413*. The switching discriminator SD responds to the second and third digits called in the same manner as before. However, as only one trunk line extending to the Grove exchange has been seized there being no, confliction between the first and second digits of Grove with any other exchange names the switching discriminator has no switching function to perform. In response to the four remaining digits 648x of the called number, the fourth selector 4E a fifth selector such as 5E and a connector H are operated to establish the connection with the line of the substation A.

It will now be assumed that the subscriber at substation A desires to call a subscriber at substation A the line of which terminates in the same exchange as that of the substation A, the directory number of this substation being CENter-2 l5x.- In response to the removal of the receiver, the primary line switch C operates to select a trunk line extending to an idle selector repeater such as 1E. In response to the first character C the wipers of the selector repeater 1E are brought opposite the second level of bank contacts and are then automatically rotated in search of an idle trunk line. In the situation shown. in the drawings, there is no other ofiice in the system having its first character in the same hole on the calling device as the letter 0. Therefore, the second level of the selector repeaters such as 1E are dead and the trunk hunting operation of this switch is without function.

Should there be other exchanges in the system having exchange names beginning with the letter C or one 1n the same finger hole of the calling device dial, the selector repeater 1E would function to select a trunk line to the QifiCe which serves as a switching center on calls from Center exchange in the districts in which the desired exchange is located. V V

hen the next two characters EN are dialled on the calling device, the wipers of the switching discriminator SD are operated to the third level of bank contacts and are then rotated into engagement with the sixth contact set. The switching discriminator SD now causes the line switch C to automatically operate to extend the connection to a fourth selector such as LE in the Center office. Should a trunk line have been seized by the selector repeater 1E on the second level, this trunk line would alsobe released by the operation of the switching discriminator in a manner similar to that already described. The fourth selector 4E a fifth selector 5E and a connector H are operated in accordance with the four remaining digits 254x to establish the connection with the line of the substation A From the above it will be seen that oflices, which in the regular Strowger trunking system would be located in the same district, may be located in different districts without regard to their names, and that a number of routes are tentatively employed to extend the connection to the desired line, the only one held being the one over which it is finally determined that the connection is to be completed.

Referring now more particularly to the detailed circuit drawings, Figs. 510, inclusive, in Fig. 5 there is shown an automatic substation A whiehis of the usual type having the calling device S. The line conductors 11 and '12 of the substation A terminate at an exchange in the individual line or trunk selecting switch C.

The line switch G is of the usual rotary type whose wipers have no normal position,

always standing in engagement, when idle, with the bank contacts associated with the trunk line last used. The line switch C, in common with a plurality of other line switches of the group has access to trunk lines extending to selector repeaterswitches, one of these trunk lines being shown in the drawings terminating in the bank contacts 28-30, inclusive, and extending to the selector repeater 1E, Fig. 5.

The selector repeater IE is of the usual Strowger type of mechanical construction. There are some differences, however, that will now be mentioned. The selector repeaters such as 1E are provided with two sets of bank contacts and an additional set of wipers. twenty levels instead of the usual ten. However, the switch shaft is only 'adapted to take ten steps and the wipers when rotated engage bank contacts in two levels. The se lector repeaters such as 1E have access to trunk lines extending to various esehanges in the different districts through secondary line switches, such'as C and C Fig. 6.

Associated with each one ofthe selector repeaters is a switchingdiscriminator, such as SD, Fig. 9. The switching discriminator SD in the present case is ofthe'same mechanical construction as a Strowger connector switch and is adapted to respond to two digits. The wipers'520 and 521of the switching discriminator SD have access to 300 bank contact sets in ten levels. The switching discriminator is adapted tocontiigl the operation of the selector repeater 1 f The secondary line switches C and C, Fig. 9, arealso permanently associated with the selector repeater 1E. These line switches are similar to the primary line switch C al ready described. The line'switch C has ac-' cess to trunk lines extending to local'fourth selectors in Center exchange, one of these being indicated under the'reference character 4E Fig. 10. The'line switch C has access to trunk lines'extending to incoming fourthselectors'in the Lombard exchange.

The trunk lines accessible to the selector repeater 1E terminate in outgoing secondary line switches, such as C and C Fig; 6. These line switches are similar to the primary line switch C and the secondary line switches C and C Thelineswitch C has access to trunk lines extendingto incoming second selectors in theGrove exchange in' district A. The secondary line switch" C has access to trunk lines extending to incoming second selectors in Main exchange of district C, one of these being shown inthe drawings terminating in bank contacts 258 260, inclusive, and extending to theincoming second selector 2E, Fig. 7.

The incoming second selector 2E,Fig. 7, is of the usual Strowger two'wiretype, and has access to trunk lines ext-endingto third selector switches in Main exchange, such as the selector 3E, Fig. 7.

The third selector 3E, the circuits ofwhich have only been indicated in the drawings,

is similar tothe second selector 2E inall respects except that in the third selectors the release trunks are extended back so as to provide a holding circuit forthe preceding'second selectors, which are adapted to seize them. The third selectors'such as 3E have access to local fourth selectors in the Main exchange, such as the one indicated by the reference character 4E and also to trunk lines extending-toincoming fourth selectors in theLarabee'exchange by way of repeaters. One of these latter trunk lines has been shown in thedrawings terminatin'g in the bank contacts 355-357, inclusive, and extending by way of' the repeater R to the incoming fourth selectors 4E, Fig. 8, in Larabee exchange.

The incoming fourth I selector 4E is similar to theincoming second selector 2E in the Main exchange and has access to trunklines extending to local fifth selectors such as the selector 5E.

TheselectorfiE, Fig. 8, is'similar' to the thirdselector' 3E already described and is adaptedto connect with trunk lines extend ing to connector-switches in the Larabeeexchange, one of these'connectors being shown in the drawings at H.

The connector H is of the usual Strowg'er' vertical and rotary type and is adapted to extend connections to local subscribers lines in the Larabee exchange- One of the'local lines, that of the substation A to which'the connector H has access is 'shown'in the drawings terminating in bank contacts 474-476, inclusive.

The substation A is similar to the sub station A; already described. The line of the substation A terminates at the exchange in the'rota'ry line switchC in addition to the multiple bank contacts of the connector switches which have access to it. The line switch C is'ide'ntical-in all' respects to the primary line switch'C, Fig. 5.

The local fourth selectors in the Main'ex change such asthe selector 4E Fig. 7, are similar to the third selectors such as 3E and have access totrunk lines extending-'to'local fifth selectors in the Main office.

These fifth selectors in turn are adapted to extend connections to local connector switches similar to theconnector'H, Fig. 10.

The connector switches, of course, have access to local subscribers lines in the'Main office in the same manner as the connector H has access to'the line of the si'ibstati'o'nA in the'Larabeeexchange.

The local fourth selectors such as 4E ,-the localfifth'select'or such as 5E and connector H, Fig; 10, in the Center exchange, are similar to corresponding selectors and connector switches that have been previously described and are: adapted to complete connections in Center exchange. r

The line of the substation A which has been'shown' accessible t'o'the connector H Fig. 10, is one of the subscribers" lines in the Center exchange.

The'substation A is similar to the: substations A and'A previously described, andthe line switch Ci' in whicli the line of" this subshown in Fig. 4. In orderto describe these operations, it will first be assumed, as in the general description, that the subscriber at substation A, whose line terminates in Center exchange, desires to connect with the line of the substation A in the Larabee exchange, the directory number of this substation being LARabee467x. In order to do this, the calling subscriber will remove his receiver and operate his calling device S in accordance with the first three letters of Larabee, LAR, and then the numbers 467x.

When the receiver is removed at substation A, a circuit for the line relay 14 of the line switch C is completed over the line conductors 11 and 12 in series, Relay 14, upon energizing, at its armature 24 completes a circuit which includes the switching relay l5 and the motor magnet 17 in series and at its armature 23 connects the test wiper 26 to the above circuit at a point midway between the switching relay and the motor magnet. The operation of the line switch C now depends upon whether the trunk line associated with the switch wipers is busy or idle. Assuming the trunk line to be busy, there will be a ground potential upon the test contact engaged by the test wiper 26, and the motor magnet 17, which is supplied with direct ground by way of the said test wiper, operates as a buzzer to advance the switch wipers 2527, inclusive, step by step until they are brought into engagement with a set of bank contactsassociated with an idle trunk line. However, we will assume in the present case, that when the receiver is removed at substation A, the switch wipers of the line switch C are in engagement with the bank contacts associated with an idle trunk line, and it will be assumed further that this trunk line is the one shown in the drawings terminating in bank contacts 28-30, inclusive. In this case the switching relay 15 is energized immediately after the energization of the line relay 14. Relay 15, upon operating, at its armature 19 places ground upon the test contact 29 of the seized trunk line and prepares a holding circuit for itself, at armatures 18 and 20 disconnectsthe line conductors 11 and 12 of the substation A from the winding of the line relay 14 and from ground, respectively, and extends them by way of wipers 25 and 27, bank contacts 28 and 30, normally closed springs controlled by armatures 65 and 66, left hand windings of the repeating coil, through the upper and lower windings of the line relay 56 of the selector repeater 1E, to battery and to ground, respecthe operating circuits of the selector repeater, and at armature 89 places ground upon the release trunk conductor 103, thereby establishing a holding circuit for the switching relay 15 of the primary line switch C. This holding circuit may be traced fromground upon the release trunk conductor 103 by Way of bank contact 29, test wiper 26, armature 19 and its front contact, through the winding of the switching relay 15, and thence through the winding of the motor magnet 17 to battery. This holding circuit serves to maintain the switching relay 15 energized throughout the connection, and it will be understood that it is established before the slow acting line relay 14 has had time to deenergize. A branch of this holding circuit also extends by way of the private normal conductor 22 to multiply connected test contacts in the banks of the connector switches which have access to the line of the substation A, whereby the said line is protected from intrusion throughout the existence of the connection.

\Vhen ground is placed upon the release trunk conductor 103, another circuit is completed which extends from release trunk conductor 103, conductor 145, through the Wind ing of the relay 504 of the switching discriminator SD, to battery. Relay 504 is energized over this circuit and, upon operating, opens the circuit of the release magnet 507 of the switching discriminator.

The calling subscriber at substation A may now operate his calling device in accordance with the first character L of the called number, whereby five interruptions are produced in the circuit of the line relay 56 of the selector repeater 1E. Relay. 56, upon deenergizing in response to these interruptions, sends five impulses of current to the vertical magnet 62 of the selector repeater over a circuit extending from ground by way of armature and its back contact, armature and ,its front contact, armature 99 and its back contact, through the winding of the slow acting low resistance relay 58,iand'thence through the winding of the vertical magnet 62 to battery. Magnet 62 operates to raise the two sets of switch wipers 120-122, inclusive, and 124426, inclusive, step by stop until these wipers are placed opposite the fifth level of bank contacts. The slow acting relay 58 is energized in series with the vertical magnet 62 and upon the first vertical step of the switch shaft when the off normal springs are shifted, completes the circuit extending from ground by way of armature 89 and its front contact, front contact and armature 92, resting contact of off normal spring 84 and the said spring, and through the winding of the stepping relay 59 to battery. Relay 59, upon operating, at its armature 93 prepares a circuit for the rotary magnet 63, and at its armature 94 establishes a locking circuit for itself extending from ground upon release trunk conductor 103 byway of front contact and armature 94, back contact and'armature 98,. working contact of ofi'normal spring 84 and said spring, and through the winding of the relay 59 to battery. WVhen the switch wipers are brought opposite the fifth level of bank contacts a cam on the switch shaft operates the spring, 71 and causes it to engage its working contact. This operation preparescertain circuits for the switching discriminator SD as will appear more fully lateri. Shortly after the termination of the vertical movement of the'switch,the slow acting 1elay58 deener gizes and a circuit is immediately closed for the rotary inagnet'63." Magnet 63, upon op erating, rotates the two sets of switch wipers into engagement'with the first set of. bank contacts onthe fifth level, and at its armature 98 opens the locking circuit of the stepping relay 59. Relay'59 immediately deenergizes, opening the circuit of the rotary magnet 63, which deenergizes also. The operation of the switch 1E now depends upon whether the trunk lines with which the switch wipers are associatedare busy or idle. Assuming that" both trunk lines are busy, there will be a ground potential upon the testcontacts engaged by the test wipers 121 and 125', and a circuit will be thereby coinpleted 'for the stepping relay 59. Upon energizing, relay 59 establishes a locking 'circuitfor'itself, as before, and'co'rn'pletes the circuit of the rotary magnet 63L- Magnet 63 operatesto advance the twosets of switch'wipers into engagement with the next two sets ofba'nk contacts on the fifth levels, and at armature 98 opens the circuit of the stepping relay 59. Relay 59 thereby 'deenergizes opening the circuit of the ro-' tary magnet 63 which deenergizes also. It' willbe notedthat both the test wipers 121 and;125 associated 'with each of the two sets ofwipers are connected to the steppingrelay 59'. Therefore, the relay 59'and the rotary magnet63 will continue to operate alternately until both sets of switch wipers are associated with an idle trunk line. It willb'e assumedthat the switch wipers 120122, inclusive, are rotated into en agement with the bankcontacts 130-132, inclusive, andthat the switch wipers 124 126, inclusive, are rotated into engagement with the bankcontacts 140142, inclusive, and it will be assumed further that the trunk lines associated with these contact sets are idle. Under these conditions, immediately upon the two sets of wipers being brought into engagement withthese bank contacts, the switching relay 60, which has been maintained short circuited during the preceding operations of the switch, isenergized in series with the stepping relay 59. The latter relay'does not operate in series with the switchingrelay, dueto the comparatively high resistance of the winding. of the relay 60.- Relay 60, upon energizing, at armature 99 opens a'point in'the 'circ uit of' the vertical magnet 62, at the front contact of this armature prepares an operating circultextending to the swltching dlscrnnand96 places a bridge across the wipers 120 and 122 and 124' and 126, respectively,

through contact springs 86 and 87 ofthe line relay 56. The line relay 201 of the secondary line switch C and the line relay 241'of the secondary line switch 0 are immediately energized. Thecircuit of the line relay 201 may be traced as follows: from ground by way of armature 89 and its front contact, front contact. and armature 97 armature 77 and its back contact, test wiper 121, test contact 131,

trunk conductor 16, armature 206 and its back contact, and through the winding of theline relay 201 to battery. Relay 201, upon energizing, at-armature 235'completes acir= cuit for the switching relay 200 in series with the motor magnet'202, atits-ai-mature 234 connects the test wiper 215' to the-abovecir cuit at a polnt midway between the switching relay and the motor magnet. The trunk se- 7' lecting operation of thesecondary line switch C now takes place in the same manner as was described in connection 5 with the operationof the primaryline switch C. When an idle trunk line is found, which, we will assume, is the trunk line terminating "in bank contacts 218220, inclusive, the'switching relay 200 is" immediately energized. It will [be remembered that thetrunk lines accessible to the secondary line switch 0 {extend to=incoming second selectors in the-Grove exchange. Only two line conductors 'ofthetrunkline extend to the incoming second-selector, as one of the functions ofthe selector repeater 1E is to repeat'iinpulses over the trunk line. -The'; test contacts associatedwith these trunk lines in the bank of the secondary rotary lineswitches such as C are multipled'together; Whena trunk line is seized, a group potential placed upon the multipledtest' contact', be ing fed ahead from the operatedfselector' re peater. The relay 200 of the secondary line switch G upon operating, at its armature 207 places ground upon the test contact 219 to busy the seized trunk line, at-armatures 205 and 208 extends the conductors 160 and 172 ofthe trunk linebyway of 'wipers 214 and 216, bank contacts 218 and 220, over trunk conductors 232 and 234 to the upper and lower. windings of line relay of the incoming fourth selector in the Grove exchange, which has been indicated in the trunking diagram 1 of Fig. 4, under the reference character 2E.

The line-relay 2410f. the secondary line. v

switch C which has been seized by the wipers 124-126, inclusive, of the selector repeater 1E, is energizedover a circuit similar to the circu-it of the line relay 201 of the line switch- C i Upon energizing, relay 241': initiates the trunk hunting operation of the secondary line line relay 300 of the incoming second selector 2E, Fig. 7, in Main office of district C.

The line relay 300 of the second selector 2E in Main exchange and the line relay of the incoming second selector in Grove exchange, are energized through contacts on the line relay 56 of theselector repeater 1E in Center ofiice. The line relay of the incoming second selector 2E in the Grove exchange, closes a circuit for its associated slow acting release relay, the latter operating to prepare the operating circuits of the incoming second selector in the usual manner. The line relay 300 of the second selector 2E, upon operating, closes the circuit of the slow acting relay 301. Relay 301, upon operating, at armature 309 opens a point in the circuit of the release magnet 31, at the front contact of this armature prepares a circuit for the vertical magnet 312, and at armature 308 prepares a circuit for the switching relay 304. p

The calling subscriber at substation A may now operate his calling device in accordance with the second character A of the called number. By this operation, one interruption is produced in the circuit of the line relay 56 of they selector repeater E. By the operation of this relay, this interruption is repeated to the line relay 300 of the incoming second selector 2E the Main office. and to the line relay of the second selector 2E in the Grove oflice. The operation of the line relay 300 of the selector 2E causes one impulse of current to be sent to the vertical magnet 312 in series with the slow acting relay 302. Magnet 312 operates to raise the switch shaft carrying the wipers 330332, inclusive, opposite the first level of bank contacts. The slow acting relay 302 is energized in series with the vertical magnet and, upon the first vertical step of the switch, when the off normal springs are shifted, completes a circuit for the stepping relay 303. Relayi303, upon operating, closes a locking circuit for itself at armature 318, and at armature 319 prepares a circuit for the rotary magnet 320. A short interval after the termination of the vertical movement of the shaft, the slow actlng relay 302 deenergizes and completes the circuit of the rotary magnet 320. Magnet 320 operates to rotate the switch wipers 330332, inclusive, into engagement wlth the first set of bank contacts 'on the first level,

and opens the circuit of the relay 303 which the switching relay 304 isenergized in series I with the stepping relay 303. Upon operating, relay 304, at armature 323 places ground upon the test contact 335 of the seized trunk line in order to busy it, at armatures 322 and 325 disconnects the trunk conductors 274 and 275 from the windings of the line relay 300 and at the front contacts of these armatures extends them to the line relay 340 of the third selector 3E. V

The line relay 340 of the third selector 3E is immediately energized and operates to complete the circuit of the slow acting relay 341. Relay 341 operates to prepare the operating circuits of the third selector 3E and at armature 344 places ground upon the release trunk, whereby a holding circuit is established for the switching relay 304 of the incoming second selector 2E. r

In a like manner, in response to the second character A, the wipers of the incoming second selector 2E Fig. 4,.in the Grove exchange, are raised to the second level of bank contacts and arethen rotated until an idle trunk line is selected extending to a third selector, which it will be assumed, is the selector 3E shown in the trunking diagram, Fig. 4.

The line relay of the third selector 3E is immediately energized and closes the circuit of the associated slow acting release relay. Bythe operation of the release relay, the switch circuits are prepared and a holding circuit is established for the incoming second selector 2E I The selector repeater 1E, in addition to repeating the interruptions produced by the calling device to the line relays of the selectors in Main and Grove exchanges, sends an impulse of current to the vertical magnet of the switchingdiscriminator SD, Fig. 9, over a circuit extending from ground by way of armature and its back contact, armature and its front contact, armature 99 and its front contact, conductor 150, oflt' normal springs 515 and 516, through the winding of the slow acting relay 503, and thence through the winding of the vertical magnet 506 to battery. The vertical magnet 506 of the switching discriminator SD operates to raise the switch shaft, carrying the wipers 520 and 521 opposite the first level of bank contacts. With the first upward movement of the switch shaft, the off normal springs are operated and the previously traced vertical magnet circuit is opened at the spring 516 but is immediately closed at the off normal'springs 515 and 517 and at armature 512 on the slow acting relay 503, which is energized in series with the vertical magnet. Shortly after the termination of the vertical movement of the switch, the slow acting relay 503 deenergizes and transfers the. operating circuit to the rotary magnet 505.

The-calling subscriber may now dial the third character R of the desired number. This operation produces seven interruptions in the circuit of the line relay 56 of the selector repeater 1E. By the operation of relay 56 these interruptions are repeated to the line relay 340 of the third selector 3E in the Main othce, and to the line relay of the third selector SE in the Grove office. The line relay 340 of the selector 3E controls the vertical movement of the switch, whereby the wipers 350352, inclusive, are brought opposite the seventh level of bank contacts. Immediately after the vertical movement of the switch, its trunk hunting rotary movement is initiated and continues, in the same manner as has been described, until an idle trunk is found. e will assume that the trunk line shown in the drawings terminating in bank cont-acts 355-357, inclusive, is the one selected. The switching relay 342 is then energized, and operates to extend the connection to the line relay 370 of the repeater R.

The line relay 370 immediately energizes and closes the circuit of the slow acting relay 371 at armature 375, and at armature 376 places a bridge across the trunk conductors 379 and 380, whereby a circuitis completed for the line relay of the incoming fourth selector 4E, Fig. 8, in the Larabee exchange. The line relay of this selector operates to close the circuit of the slow acting release relay. The latter relay prepares the operating circuit of the selector in the usual manner. In the Grove exchange the wipers of the third selector 3E are raised to the seventh level of bank contacts and are then rotated in search of an idle trunk line. As the third digit determines that a subscriber whose line terminates in the Larabee exchange is being called the trunk hunting operation of the selector 315 is without function.

The line relay 56 of the selector repeater 1E, in operating in response to the third character R, sends seven impulses of current to the rotary magnet 505 of the switching discriminator SD. Magnet 505 rotates the switch shaft carrying the wipers 520 and 521 step by step until the said wipers are brought into engagement with the seventh set of bank contacts on the first level. The slow acting relay 502 is energized in series with the rotary magnet 505, and operates at its armature 511 to remove round from the wipers 520 and 521 of the switching discriminator.

Shortly after the rotary movement of the switch terminates, a circuit may be traced extending from ground on, release trunk conductor 103, conductor 145, back contact and armature 511, through the windings of relay 501, conductor 152, shaft controlled spring 71 and its working contact on the selector repeater 1E, conductor 151, wiper 520, bank contact 17, conductor 148, and through the lower winding of'the relay 54 to battery. The relay 54 energizes over this circuit and,upon operating. at its armature 78 establishes a locking circuit for itself, at armature 77 disconnects ground from the test wiper 121 thereby opening the holding circuit of switching relay 200 of the secondary line switch C and at armatures 76 and 79 closes a new bridge across the lower set of wipers 124and 126, at the same time rendering the impulse springs 87 on the line relay 56 ineffective. The relay 501 is energized in series with the lower winding of relay 54 and operates to open the rotary magnet circuit so that the switching discriminator SD will not be operated responsive to subsequent digits dialled by the calling subscriber. The above mentioned bridge may be traced from switch wiper 124, front contact and armature96, front contact and armature 76, back contact and armature 72, back contact and armature 68, upper right hand winding of the repeating coil, through the left hand winding of the electro-polarized shunt field relay 51, lower right hand winding of the repeating coil. front contact and armature 88 on line relay 56, armature 70 and its back contact, armature 75 and its back contact. armature 79 and its front contact, to wiper 126. When the holding circuit of the relay 200 of the secondary line switch C is opened, this relay deencrgizes to disconnect the trunk conductors 160 and 162 from the trunk conductors 232 and 234 of the seized trunk line extending to Grove exchange. The circuit of the line relay of the third selector 3E in Grove exchange is accordingly opened and this relay deenergizes to bring about the release of the third selector 3E and the incoming second selector 2E Another result of the deenergization of relay 200 is that ground is disconnected from the test contacts such as 219 of the seized trunk line extending to Grove exchange, whereby the said trunk line is rendered accessible to other rotary secondary line switches in the group.

It will be remembered that the previously traced bridge across the wipers 124 and 126 includes the spring 88 on the line relay 56, whereby the switches in the Grove exchange may be controlled.

hen the calling subscriber at substation A operates his calling device in accordance withthe next digit 4, four interruptions are produced in the circuit of the line relay 56 of the selector repeater 1E and by the deenergization of this relay, are repeated to the line relay 370 of the repeater B. This relay an idle trunk line which, it will be assumed,

are the bank contacts 404-406, inclusive. Immediately upon the switch wipers engaging this set of bank contacts, the switching relay of the fourth selector 4B is energized to extend the connection to the line relay of the fifth selector The line relay of the fifth selector operates immediately to close the circuit of its associated slow acting relay. The latter relay, upon operating, establishes the usual holding circuit for the incoming fourth selector 4E and also prepares the fifth selector 5E for operation.

. The calling subscriber may now operate his calling device for the next digit of the desired number. The operation of the calling device serves to produce six interruptions in the circuit of the line relay 56 of the selector repeater 1E. The resulting deenergizations of this relay repeat the six impulses to the line relay of the repeater B. As before, the line relay 370 of the repeater R then repeats the six interruptions to the line relay of the fiftn selector-5E. The line relay of the fifth selector SE, in operating in, response to these interruptions, controls the vertical magnet in stepping the wipers 410-412, inclusive, of the fifth selector to the sixth level of bank contacts. Shortly after the termination of the vertical movement of the switch its rotary movement is initiated and takes place until the switch wipers are associated with the ank contacts of an idle trunk line. It will be assumed that the trunk line terminating in the bank contacts 414 416,inclusive, is the one selected and connected with by its selector 5E. Under these conditions, the switching relay of the selector 5E energizes to extend the connection to the line relay 421 of the connector H.

The relay 421 is immediately energized and, upon operating, at its armature 438 closes the circuit of the slow acting relay 422. Upon operating, the latter relay at armature 441 opens the release magnet circuit, at the front contact of this armature prepares the impulsing circuits of this connector, at armature 440 prepares certain circuits in the connector H, and at armature places ground upon the release trunk conductor 465, thereby establishing a holding circuit for the fifth selector 5E and the fourth exchange has been released, the operation of the armature 86 of the line relay 56 in response to the digit 6, has no function. Also, as the switching discriminator SD has performed its function, it also does not operate further.

When the calling device at substation A is operated in accordance with the next digit 7 of the desired number, seven interruptions are produced inthe circuit of the line relay 56 of the selector repeat-er 1E and, by the operations of this relay and the operations of the line relay 370 of the repeater R, these interruptions are repeated to the line relay 421 of the connector H. The line relay 421 of the connector H deenergizes a corresponding number of times, and seven impulses of current are sent to the vertical magnet 428, the first impulse traveling over the following circuit: groimd by way of armature 438 and its back contact, armature 441 and its front con tact, off normal springs 443 and 445, through the slow acting relay 423, and thence through the winding of the vertical magnet 428 to battery. The above is the circuit over which the first vertical magnet impulse is trans mitted. However, upon the first vertical movement of the switch. s iaft the off normal springs are shifted and the remaining impulses to the vertical magnet traverse a new circuit, which is the same as the one previously traced except that it includes the off normal springs 443 and 444 and armature 450 on the slow acting relay 423, this relay being now energized. The vertical magnet 428 operates to raise the switch shaft carrying the wipers 4i'0-472, inclusive, step by step until they are brought opposite the seventh level of bank contacts. The slow acting relay 423 is energized in series with the vertical magnet and, as indicated before, operates at armature 450 to maintain the circuit of the vertical magnet intact tl'lroughout the vertical movement of the switch shaft. Shortly after the cessation of the vertical movement, the slow acting relay 423 deenergizes to prepare a circuit for the rotary magnet p The calling subscriber may now operate his calling device in accordance with the next final digit of the desired number; By this operation a series of interruptions is produced in the circuit of the line relay 56 of the selector repeater 1E in the Center exchange. The line relay 56 operates to repeat these EL -s of interruptions to line relay 8'70 of the repeater B. By the operation of the latter re" this last series of interruptions is produced in the circuit of theline relay 421. The deenergization of line relay .421 in response to these interruptions sends a plurality of impulses corresponding in number to the digit dialed to the rotary magnet 435 over the following circuit: ground through armature 438 and its back contact, armature 441 and its front contact, otf normal springs 443 and 444, armature 450 and its back contact, back contact and armature 455, armature 462 and its back contact, and through the winding of the rotary magnet 435 to battery. Magnet- 4-35 operates to rotate the shaftof the switch step by step until the wipers 470-472, inclusive, are brought into engagement with the set of bank contacts associated with the line of the called substation A which, it will be assumed, are the bank contacts 474 476, inclusive. The slow acting relay 426 is energized in multiple with the rotary magnet 435, and atarmature 4'57 operates to connect the test wiper 471 to the test relay 425, and at armature 458 short circuits the contact springs comprising armature 455 and its back contact on the test relay 425, so as to main-.

tain the circuit of the rotary magnet closed while the wipers of the switch are being rotated over bank contact sets associated with busy lines. The operation or" the connector H now depends upon whether the called line is busy or idle.

Assuming first that the called line is busy, there will be a ground potential upon the test contact 475 engaged by the test wiper 471. A circuit is thereby completed for the test relay 425. Upon energizing, relay 425 at armature 454, closes a pointin its locking circuit which is completed immediately after the deenergization of slow acting relay 426, at its armature 455 opens a point in the rotary ma net circuit, and at armature 456 connects a lead from the busy signalling machine to the lower heavy talking conductor, whereby the calling subscriber is informed that the desired line is busy. The subscriber at substation A will now replace his receiver. By this operation the switches used in extending the connection to the line of the substation A in the Larabee exchange are released in a manner which will be described subsequently.

It will now be assumed that the line of the substation A is idle when the switch wipers 47 0 472, inclusive, are brought into engagement with the set of bank contacts 474467, inclusive. Under these conditions, there will be no ground potential upon the test contact engaged by the test wiper 471 and no circuit will be completed for the test relay 425. Then, shortly after the termination of the rotary movement of the switch, the slow acting relay 426 deenergizes and a circuit is completed extending from ground by way of front contact and armature 440, armature 454 and its back contact, through the upper winding of switching relay 427, back contact and armature 457, test wiper 471, test contact 475, private normal conductor, through the winding of the switching relay 480 of the line switch 0 and thence through the winding of the motor magnet 482 to battery. The switching relays 427 and 480 are energized over this circuit. However, due to a mechanical locking device controlled by the armature of the line relay 481 of the line switch C the switching relay 480 only partially operates its armat-ures, enough to disconnect the line of the called substation from its normal battery and ground connections in the line switch C In the connector H, the switching relay 427, upon energizing, at its armature 4G1 establishes a locking circuit for itself, at armature 462 opens the circuit of the rotary magnet, at armature 460 places di 'ect ground upon the test contact 475, and at armatures 459 and 463 completes the signalling circuit. Ringing current from the generator GEN is now intermittently projected out on the called line until the receiver is removed at substation A hen the receiver is removed at substation A a circuit is completed for energizing the ring cut off relay 424. Upon operating, relay 424, at its armature 452 establishes a locking circuit for itself, at armatures 451 and 453 disconnects theringing current, and at the front contacts of these armatures finally completes the talking connection. Talking cur-' rent is supplied to the called substation from the windings of the back bridge relay 420 which is, 01 course, energized over the called line loop. Relay 420, upon energizing, at its armatures 436 and 437 operates to reverse the connections to the line relay 421 with regard to the trunk conductors 379 and "380, wh reby the direction of current flow over these conductors is reversed. As the current flow in the left hand winding of the electro-polarized shunt field relay 372 in the repeater R is nowin thesame direction the current flow in its polarizing left hand winding, the armatures 396 and 397 of this relay-will be attracted and the direction of current flow over the trunk conductors 274 and 275 will be reversed, thereby causing the electro-polarized shunt field relay 51 in the selector repeater 113 to operate. Upon operating, the electro-polarized relay 51 closes the circuit of the reversing relay 50. The latter relay, upon operating, reverses the current flow over the calling line for supervisory or metering purposes, as the case may be. Conversation may now take place between the calling subscriber at substation A in the Center exchange and the called subscriber in substation A in the Larabee exchange, talking current being supplied to the substation A from the windings of the line relay 5G in the selector repeater 1E. The talking circuit may be readily traced, as it is indicated by the heavy conductors in the drawings.

When the conversation is terminated, the calling subscriber will replace his receiver on the switch-hook. This operation opens the circuit of the line relay 56 of the selector repeater 1E, which deenergizes, opening the circuit of the slow acting relay 57 and also the circuit of line relay 370 of the repeater R in the Main exchange. The slow acting relay 57, upon deenergizing, closes the circuit of release magnet 61 at armature 90, and at armature 89 removes ground from the release trunk conductor 103, whereby the holding circuit of the switching relay 15 of the primary line switch C is opened. The deenergization of relay 15 restores the primary line switch to normal. The removal of ground from the release trunk conductor 103 also opens the circuit of relay 504 in the switching discriminator SD. Belay 504;, upon deenergizing, closes the circuit of the release magnet 507. Magnet 507 of the switching discriminator operates to restore the switching disc-riminator to normal. The circuit of the re- ,lease magnet being opened by oil normal the switch shaft carrying the two sets of switch wipers to normal in the usual manner. Another result of the deene zation of the slow acting relay 57 is that ground is removed from the switch wiper 125 whereby the hold ing circuit of the switching relay 2 10 of the secondary line switch C is opened and this relay deenergizes to restore this switch to nor mal.

As the circuit of the line relay 370 of the repeater R in Main exchange has been opened by the deenergization of the line relay 56 of the selector repeater 1E, this relay deenergizes and, upon so doing, at armature opens the circuit of the slow acting relay 371, and at armature 376 opens the circuit of the line relay 421' of the connector H in the Larabee office. The deenergization of slow actingrelay 371 in the repeater R removes ground from the release trunk conductor 361 thereby opening the holding circuit of the switching relays 30 1- and 342 of the second selector 2E and the third selector 3E, respectively, in Main otlice. By the energization of these switching relays the respective switches are restored to their normal position.

The circuit of the line relay 421 of the con nector H being opened, this relay deepergizes, and opens the circuit of its associated slow acting release relay 422. The latter relay, upon retracting its armat-ures, closes the circuit of the release magnet 429 of the connector H at armature 441, and at armature 189 opens the locking circuit of the ring out off and switching relays 424 and 127. respec tively, and also at this same armature removes ground from the release trunk conductor 465, whereby the holding circuits of the fourth selector 4E and the fifth selector 5E in the Larabee exchange are opened. By the deenergizations of the switching relays in these switches the fourth and fifth selectors are restored to normal. The release magnet 1-29 of the connector H operates to release the switch shaft of the connector, the circuit of the release magnet being opened at oif normal springs when the switch shaft reaches its normal position.

The release of the connector H causes ground to be removed from the test contact 475 of the called line, whereby the circuit of the switching relay 480 of the called line switch C is opened. Relay 480, upon deenergizing restores the line switch C to normal.

In the above manner all the apparatus involved in the extension of a connection from a subscriber at substation A in the Center exchange to the line of the substation A in the Larabee exchange is released.

It will now be assumed that the calling subscriber at substation A, instead of desiring to call the subscriber at substation A in the Larabee exchange, desires to extend a connection to the subscriber at substation A in the Main exchange whose number is MAIn 736x. Under these conditions, the subscriber at substation A instead of operating his calling device for the third character R, as was previously the case, will operate his call ing device for the character I. By this operation one interruption is produced in the circuit of the line relay 56 of the selector repeater 1E, and by the deenergization of this relay is repeated to the line relay 840 of the third selector 8E in the Main exchange. The line relay 340 of the third selector 3E controls the operation of the switch, whereby the wipers 350353, inclusive, are brought opposite the first level of bank contacts. Shortly after the termination of the vertical movement of the switch. the trunk hunting opera tion takes place, whereby the switch wipers are brought into engagement with a set of bank contacts associated with an idle trunk line, which, we will assume. are the bank contacts 358360. inclusive. Immediately upon the switch wipers engaging this set of bank contacts, the switching relay 342 of the third selector 3E is energized and operates to extend the connection to the line relav of the fourth selector 4E in the Main oflice.

In the selector repeater 1E, the deenergization of the line relay in response to the third character I causes one impulse of current to be sent to the rotary magnet 505 of the switch ing discriminator SD in series with the slow acting relay 502. The rotary magnet 505 of the switching discriminator operates to rotate the switch shaft carrying the switch wipers 520 and 521 into engagement with the first set of bank contacts on the first level. Upon the deenergization of the slow acting relay 502, which is energized in series with the rotary magnet, ground. is placed upon the wiper 520 of the switching discriminator SD, whereby a circuit is completed extending from the said wiper, bank contact 11, conductor 148,

and through the lower winding of relay 54 

